1. Field of the Invention
The present invention relates to a fluid bearing, a motor, and a polygon mirror scanner motor, and more particularly to a fluid bearing, motor, and polygon mirror scanner motor in which the reliability can be improved.
2. Description of the Related Art
A motor may use a ball bearing or a fluid dynamic pressure bearing in order to rotatably support a rotor with respect to a stator. Among such bearings, a fluid dynamic bearing is configured so that a shaft is supported by using the dynamic pressure of a fluid which is generated during rotation. In a fluid dynamic bearing, grooves for generating a dynamic pressure are arranged in at least one of an outer circumferential portion of a shaft, and an inner circumferential portion of a sleeve into which the shaft is inserted. In such a fluid dynamic bearing, a radial bearing which receives a load in a radial direction of the shaft is formed from dynamic pressure generated by the fluid that is interposed between the shaft and the sleeve. Furthermore, a thrust receiving member is placed in a lower end portion of the shaft, or a position opposed to a lower end portion of the shaft, and a thrust bearing which receives a load in the thrust direction of the shaft is formed by a dynamic pressure generated in grooves for generating a dynamic pressure that are arranged in at least one of a lower end portion of the shaft, and the thrust receiving member. There is another known structure, in which a radial bearing is combined with a thrust bearing which receives a load, and in which a groove for generating a dynamic pressure is not placed in a lower end portion of a shaft, a spherical portion is disposed therein, and a thrust plate not having grooves for generating a dynamic pressure is placed in a position opposed to a lower end portion of the shaft.
In a fluid dynamic bearing, when air bubbles (air) exist inside the bearing, reduction of the generated dynamic pressure or the like occurs, thereby impairing bearing performance characteristics and bearing life. Therefore, several methods (degassing methods) of preventing air bubbles from existing in a bearing have been proposed.
For example, JP-A-06-066315 discloses a dynamic bearing rotating device in which a rotation shaft and a sleeve are rotatably fitted to each other, a dynamic thrust bearing is formed by a shallow groove disposed in a thrust plate, and the inner circumferential surface of the sleeve, and a dynamic radial bearing is formed by herringbone-like shallow grooves disposed in the outer circumferential surface of the rotation shaft, and the inner circumferential surface of the sleeve. In the dynamic bearing rotating device, escaping portions are formed in the inner circumferential surface of the sleeve, and tapered surfaces having an angle of 30 deg. or less are disposed in the escaping portions.
JP-A-2006-275077 discloses a dynamic bearing device which includes a bearing sleeve having: two herringbone grooves; oil sump step portions that are formed in the both ends of the herringbone grooves, respectively; and spiral grooves that are formed in the vicinities of the outlets of the bearing, respectively, and in which the pattern widths of the herringbone grooves and the spiral grooves, and the lengths of the oil sump step portions are set to have predetermined relationships. In the dynamic bearing device, an annular escaping portion is formed between the two herringbone grooves of the bearing sleeve.
JP-A-2000-346075 discloses a fluid dynamic bearing including: a stator having a sleeve; a rotor having a rotation shaft which is rotatably supported by the sleeve: a dynamic pressure generating groove which is formed in one of the sleeve and the rotation shaft: and oil filled into the sleeve. In the fluid dynamic bearing, a tapered surface is disposed in a lower end portion of the shaft.
FIG. 12 is a sectional view schematically showing a step of assembling a conventional fluid dynamic bearing.
As shown in FIG. 12, in the step of assembling a conventional fluid dynamic bearing, a shaft 113 is inserted in a state where a lower end portion of a bearing hole 141 of a sleeve 132 is covered by a thrust cover 133 and a thrust plate 134, and oil 131 is filled into the bearing hole 141. thereby assembling the fluid dynamic bearing.
However, there is a tendency that the maximum value R102 of the liquid surface curvature radius of the oil 131 is reduced by the surface tension of the oil 131. During the insertion of the shaft 113, therefore, a situation easily occurs where the side surface of the shaft 113 is in contact with the oil 131 before the lower end surface of the shaft 113 becomes in contact with the oil 131, the oil 131 ascends the inner wall surface of the bearing hole 141, and the air is trapped at a lower portion U of the shaft 113. As a result, the air is not sufficiently evacuated and remains in the oil 131, thereby causing the reliability of the fluid dynamic bearing to be reduced. The reduction in reliability due to residual air in oil causes problems not limited to only fluid bearings that generate dynamic pressure, but is common to all kinds of fluid bearings. This problem can be solved to some extent by disposing a tapered surface in a lower end portion of a shaft. However, this countermeasure cannot achieve a sufficient effect.